Distributed optical fiber sensor with controlled response

ABSTRACT

Distributed fiber optic chemical and physical sensors provide a relatively highly uniform response over the length of the fiber by, for example, varying such properties as the core/cladding index of refraction ratio to compensate for the non-linearity in sensitivity due to the loss of higher order modes in multi-mode fibers. The variation of the ratio changes the absorption coefficient of the fiber and can be used to compensate for any non-linearity in response. Other techniques for compensation also are disclosed.

FIELD OF THE INVENTION

This invention relates to optical fiber sensors and more particularly tosuch sensors which include materials in the coatings and/or claddings ofoptical fibers which produce changes in the characteristics of the lightcarried by the fiber responsive to the presence of a material or fieldto be detected.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,834,496 issued May 30, 1989 discloses distributed fiberoptic chemical sensors. Such a sensor comprises an optical fiber with acore and a cladding which is permeable. The cladding, or a coating onthe cladding, includes a composition, the optical properties of whichare altered in the presence of a material to be detected. The light(i.e. the wavelength or the intensity of the light) transmitted throughthe core of the fiber is a function of the change in optical propertiescaused by the interaction of the composition included in the permeablecoating with the material to be detected. The change in opticalproperties may comprise (for example) a change in the index ofrefraction (or indices of refraction differential), or an increase ordecrease in the optical absorbance or fluorescence of the composition.

The above-noted patent discloses materials, compositions, sensingparameters and examples of such sensors, all of which are useful inaccordance with the principles of this invention. In addition,distributed fiber optic sensors for physical properties, such astemperature or pressure, can also be fabricated based on similarproperties, e.g., temperature-induced or pressure-induced changes in therefractive index, optical absorbance, or fluorescence of a cladding orcoating material applied to a light-guiding core.

Unfortunately, such optical fibers are characterized by spatialtransients for transmitted light which causes the sensitivity ofchemically or physically sensitive fibers to vary from point to pointalong the fiber. This is particularly true of multi-mode fibers withlossy (e.g. absorber-doped) coatings where light does not reachequilibrium for a considerable distance. Such a varying response is dueto a spatial transient, associated with the existence of radiationmodes, and a stronger attenuation of higher order bound modes.Accordingly, sensor response over the length of the fiber is notconstant.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the principles of this invention, the fraction oflight lost per unit length may be controlled by controlling the physicaldesign of the fiber. One use for this invention is to compensate for thespatial transient in distributed fiber optic sensors, wherein a constantsensitivity over the length of a distributed fiber sensor is achieved byintroducing a change in the light-guiding characteristics of the fiberto compensate for the effects of spatial transients. Thus, for example,in one embodiment, a loss-compensated distributed optical fiber sensorcomprises a core and a cladding where the core to cladding refractiveindex differential increases with length to compensate for the loss ofhigher order modes in the (multi-mode) fiber. As a result, a highlyuniform, distributed sensor is realized.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic illustration of an optical fiber.

FIG. 2 is a graph of power transmitted vs. fiber position for prior artapparatus; and

FIG. 3 is an illustrative embodiment in accordance with the principlesof this invention; and

FIG. 4 is a graph of refractive index variation with distance from thelaunch end of a fiber in accordance with the principles of thisinvention.

FIG. 5 is a plot of power transmitted vs. fiber position that resultswhen light is launched into a fiber the refractive index Of which variesas shown in FIG. 4.

FIG. 6 is a block diagram of a system using the fiber of FIGS. 1 and 3.

FIG. 7 is a block diagram of a further system using the fiber of FIGS. 1and 3.

FIG. 8 is a plot of energy loss vs. fiber position that results whenlight is launched into the fiber of FIG. 5 when a section of the fiberis exposed to a target chemical substance.

FIG. 9 is a plot of chemical concentration vs. fiber position that canbe derived from FIG. 8.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

Multi-mode absorption based distributed sensors have an output responsegiven by $\begin{matrix}{P \approx {P_{O}\frac{V}{\alpha\quad l}}} & (1)\end{matrix}$as is well known. In the equation, V is the fiber number given byV=ka{square root}{fraction (n _(co) ² −n _(cla) ² )},  (2)where Po is the power at the input end of the fiber, P is the power at adistance l from the input end, a is the core radius, α is the absorptioncoefficient of the cladding, and n_(co) and n_(cla) are the core andcladding refractive indices. Equation (1) is the generic response, whichcan be observed as a change in total transmitted light intensity using asimple detection scheme, observed as a plot of backscattered powerversus length in an optical time domain reflectometer (OTDR), of amulti-mode optical fiber with an absorbing cladding. It is clear thatthe power lost per unit length at each position along a distributedoptical fiber sensor is different from the power lost per unit length atany other position along the fiber.

FIG. 1 shows a representative distributed optical fiber sensor. Thepower can be calculated for each of the positions from the input (di) tothe output (do) and at any of the positions d2, d3, d4, d5, d6 - - - inbetween. Accordingly, the power along the length of the fiber is givenby: $\begin{matrix}{P = {P.{\exp\left( {- {\sum\limits_{\lambda = l}^{N}{\int_{O}^{P}{{\alpha(z)}{\eta_{i}(z)}{\mathbb{d}z}}}}} \right)}}} & (3)\end{matrix}$

In accordance with the present invention, a distributed, multi-modeoptical fiber sensor can be made to exhibit a linear response in the dBscale. A linear response to changes in absorption may be achieved bycompensating for the absorption fall off of FIG. 2 by:

-   -   a) increasing the core/cladding refractive index ratio along the        fiber length    -   b) increasing the core diameter along the fiber length    -   c) increasing the absorption coefficient of the sensor material        in the cladding along the fiber length    -   d) increasing the absorption coefficient of the cladding along        the fiber length    -   e) increasing the scattering coefficient of the core along the        fiber length    -   f) varying the parameters (a, b & c) together or in varying        combinations

FIG. 2 is a plot of power radiated per unit length versus fiber length lfor a typical multimode fiber. The absorption coefficient clearly fallsoff. The absorption coefficient change is due to the fact that all modesin an optical fiber are attenuated differently. Modes closer to cut off(i.e., modes that have an angle of incidence in the core/claddingboundary closer to 90 degrees) are attenuated quicker than modes farfrom cut off. In order to obtain an even attenuation as a function oflength, coupling from lower order modes to higher order modes mustoccur. Even attenuation is achieved, for example, by increasing the corediameter as shown in FIG. 3 or by varying the core/cladding refractiveindex ratio or by increasing the absorption coefficient or scatteringcoefficient along the length of the fiber to compensate for the lowerattenuation rate of modes that are increasingly far from cut off.

FIG. 4 is a plot of nco/ncia versus fiber length for a multi-mode,distributed sensor optical fiber in accordance with the principles ofthis invention. It is clear that an optical fiber so designed exhibits apower loss which is linear over the length of the fiber, for thecombined effect of the spatial transient of FIG. 2 and thelength-variation of fiber physical parameters (e.g., the refractiveindex, as shown in FIG. 4) is to linearize the power versus length curve(FIG. 5), thus creating a fiber sensor the sensitivity of which does notvary as a function of length.

FIG. 6 is a block diagram of sensor apparatus employing a fiber opticsensor of the type shown in FIG. 4. The fiber 100 includes a core 125transmissive of electromagnetic radiation (e.g. light) and a cladding orsheath 126 of a permeable material. The index of refraction of sheath126 is lower than that of core 125, and may vary with length. The fibermay include a coating 127 which also would be of a permeable material.

A reactant to be detected, causes a change of, for example, theintensity of light back scattered toward the end into which light islaunched. The input light is launched, and the output light is detectedby detector 131 as noted herein before.

Light energy is introduced at the input end of the fiber by,illustratively, a laser 132 and the properties of the light energy (e.g.intensity, wavelength - - -) are modified by interaction betweenmaterial in the fiber sheath and the reactant to generate backscatteredlight characteristic of the reactant and the interaction with the sheathmaterial.

The importance of structuring the fiber so that this interaction isconstant over the length of the fiber is most clear in applicationswhere the presence of a test chemical (reactant) may be at any pointalong the fiber and a cumulative effect over the length of the fiber isnot an issue. The use of an ODTR, as shown in FIG. 7, permits theposition of the reactant along the fiber to be determined as indicatedin FIGS. 8 and 9. In addition, such a loss-compensated sensor fiber canbe used to accurately measure average chemical concentration over theentire fiber length by measuring the total light loss induced by thepresence of the chemical using the system of FIG. 6. This is notpossible for fibers the power versus length curve of which is nonlinear(e.g. FIG. 2). An optical fiber structured in accordance with thisinvention to exhibit a length-invariant response to such interactions isalso advantageous for illumination applications and for applications inwhich an accumulated response is desired as in U.S. Pat. No. 4,321,057issued Mar. 23, 1982 and in the above noted Pat. No. 4,834,496 where theportion of the length of fiber affected may be compared to somethreshold for triggering an alarm.

An optical fiber having a core of F-2 Schott glass with a diameter of100 micrometers and a cladding of polymer with a thickness of 20micrometers was fabricated and a twenty meter length of the fiber wastested at 850 nm (active wavelength) as a moisture sensor. A referencewavelength of 1300 nm also was used. A dry reference measurement wasmade and then increasing lengths of 10 cm, 20 cm, and 50 cm were wetted.A final reading was taken after allowing the 50 cm wet length to air dryfor one hour. At 1300 nm only a slight change (<1 dB) in outputintensity occurs after an input signal propagates more than 10 metersalong the fiber. A significant change of 2.4 dB is detected at 850 nm.The OTDR was able to determine the location of the moisture site towithin 1 cm using 850 nm light and the 1300 nm light was capable ofbeing used to compensate for transmission charges due to effects otherthan moisture.

A PH-sensitive fiber was also tested, in a manner similar to that forthe moisture-sensitive fiber, immersed in PH-7 and PH-10 buffersolutions. At 1300 nm there was virtually DO change in output intensity.At 850 nm a substantial change of 1.2 dB occurred. The OTDR was able tolocate the position of the buffer solution along the fiber to within 2cms at 850 nm, and again the 1300 nm signal could be used as a“reference”.

The ability to structure a fiber to control the fraction of light lostper unit length in accordance with the principles of this invention alsopermits a fiber to be constructed to provide an arbitrary power lossprofile along the fiber. By so doing, a fiber sensor may be providedwhich is particularly sensitive at a predesignated position in thefiber.

1: An optical fiber, said fiber having a core and a sheath, said fiberhaving at least one parameter that varies from an input end of saidfiber to an output end thereof in a manner to maintain constant powerloss per unit length over the length of said fiber. 2: An optical fiberas in claim 1 wherein said core is fabricated in a manner to besensitive to a target chemical. 3: An optical fiber as in claim 1wherein said sheath includes a cladding and said cladding is fabricatedin a manner to be sensitive to a physical quantity. 4: An optical fiberas in claim 1 wherein said core is fabricated in a manner to besensitive to a physical quantity. 5: An optical fiber as in claim 1wherein said sheath includes a cladding and said cladding is fabricatedin a manner to be sensitive to a target chemical. 6: An optical fiber asin claim 1 wherein said one parameter comprises an increase in thediameter of said core from said input end to said output end. 7: Anoptical fiber as in claim 1 wherein said sheath includes a cladding andwherein said one parameter comprises the core/cladding refractive indexratio. 8: An optical fiber as in claim 1 wherein said one parametercomprises an increase in the absorption coefficient of said fiber fromsaid input end to said output end. 9-16. (canceled) 17: An opticalfiber, said fiber having a core and a cladding, said cladding beingfabricated to be sensitive to a physical quantity, said fiber having atleast one parameter that varies from an input end to an output end in away calculated to make the power loss vary in a controlled way over thelength of the fiber. 18: An optical fiber as in claim 17 wherein saidone parameter comprises an increase in the diameter of said core fromsaid input end to said output end. 19: An optical fiber as in claim 17wherein said one parameter comprises the core/cladding refractive indexratio. 20: An optical fiber as in claim 17 wherein said one parametercomprises an increase in the scattering coefficient of said fiber fromsaid input end to said output end. 21: A distributed fiber optic sensorcomprising a multi-mode fiber having a core and a permeable cladding,said cladding including a composition responsive to an external materialto generate a light signal characteristic of that response, said fiberhaving at least one parameter that varies as a function of positionwithin the fiber to compensate for any non-linear power loss over thelength of said fiber. 22: An optical fiber as in claim 21 wherein saidone parameter comprises an increase in the diameter of said core fromsaid input end to said output end. 23: An optical fiber as in claim 21wherein said one parameter comprises the core/cladding refractive indexratio. 24: An optical fiber as in claim 21 wherein said one parametercomprises an increase in the absorption coefficient of said fiber fromsaid input end to said output end. 25: An optical fiber as in claim 21wherein said composition is characterized by an increase in scatteringcoefficient from an input to an output end of said fiber. 26: An opticalfiber as in claim 21 also including a light sensor at an output endthereof. 27: An optical fiber as in claim 26 said fiber having a lightsource at an input end thereof.